On-demand movies ordered but won't play - get black screen

The last few times we're ordered a movie from Xfinity on-demand, it says thanks for the order, acts like it will play the movie, but all we see is a black screen. And no sound. Can't fast-forward through it either. This has happened three times now. We've called, they've reset the box, we've unplugged the box to reset it... nothing works. This is the 3rd or 4th box we've been on because of other past problems like not getting certain channels in the subscription. Please advise.